1. Field of the Invention
The present invention relates to a photovoltaic power generating apparatus, photovoltaic power generating system and method of producing the photovoltaic power generating apparatus.
2. Related Background Art
In recent years, with the emergence of serious problems such as global warming due to emission of carbon dioxide, etc., caused by use of fossil fuel and pollution with radioactivity by accidents at atomic power plants and nuclear waste, there is a glowing concern about global environment and energy. Under such circumstances, solar photovoltaic power generation using solar radiation, geothermal power generation using geothermal power and wind power generation using wind power, etc., are being put to practical use worldwide as inexhaustible and clean energy sources.
Of these energy sources, there are various modes of solar photovoltaic power generation using a solar cell according to an output scale from several W to several thousand kW. A typical system using a solar cell is a photovoltaic power generating system that converts DC power generated by the solar cell to AC power (DC-AC conversion) by an inverter, etc., and supplies the power generated to a load of a demander or a commercial power system (hereinafter simply referred to as “system”).
FIG. 2 shows a schematic configuration of a conventional, general photovoltaic power generating system. As shown in the figure, the photovoltaic power generating system 8 generally uses a solar cell module 6 made up of a plurality of serially connected solar cell elements as a unit, forms a solar cell string 7 (also called as a “solar cell array”) made up of a plurality of the serially connected solar cell modules 6, further forms a solar cell array made up of a plurality of those solar cell strings 7 connected in parallel, collects DC outputs from the solar cell arrays by a current collection box 9, converts the collected power to AC power by an inverter 3 and interconnects the AC power with a load 4 or commercial system 5. In such a photovoltaic power generating system 8, when outputs of the plurality of solar cell strings 7 vary from one string to another due to influences such as a variation in output characteristics of the solar cell or partial shadows of buildings, etc., the photovoltaic power generating system 8 may not be able to operate at optimal power points.
In order to cope with this problem, Japanese Patent Application Laid-Open No. 2000-112545 discloses a photovoltaic power generating system which provides a DC-DC converter through a connection box for each solar cell array, inputs DC output power to an inverter all together and coverts the DC power to AC power. In this configuration, each DC-DC converter performs maximum power point tracking control over the solar cell array connected thereto, and thereby improves the accuracy of the maximum power point tracking control over the photovoltaic power generating system.
Furthermore, Japanese Patent Application Laid-Open No. H08-70533 discloses the possibility of increasing or decreasing an amount of power generated by a solar cell at low costs by providing an inverter for each solar cell array, solar cell module or solar cell element, thereby reducing variations in output or difference in power efficiency due to partial shadows among solar cell arrays, solar cell modules or solar cell elements and providing an inverter for each solar cell module or solar cell element.
However, in the photovoltaic power generating system described in Japanese Patent Application Laid-Open No. 2000-112545 which inputs DC outputs of the solar cell arrays or solar cell modules to a DC-DC converter, it is necessary to serially connect a plurality of solar cell elements to create a solar cell module.
Generally, creating a solar cell module requires quite many steps such as a cutting step for dividing a photovoltaic layer stacked on a substrate into solar cell elements, an end etching step for providing a non-power generating area for insulating from one solar, cell element to another, a step of serially connecting the solar cell elements one by one using a wiring member such as an inter-connector, a step of connecting a bypass diode to reduce influences of partial shadows, a step of coating a group of serially connected solar cell elements and a step of fitting a frame at an end of the coated group of solar cell elements, and thereby takes time and uses costly members, which constitute factors responsible for increasing the price of a photovoltaic power generating apparatus.
Especially when a solar cell module having a large area is produced, the step of serially connecting many solar cell elements takes time and trouble, which constitutes a serious problem in producing a solar cell module having a large area.
Furthermore, a configuration of a plurality of solar cell elements serially connected using a wiring member such as an inter-connector requires a gap to insert the inter-connector between solar cell elements, the number of such gaps increases as the number of serially connected solar cell elements increases, which entails an increase in the non-power generation area not used for power generation in the solar cell module. As a result, the efficiency of area power generation of the solar cell module is reduced.
In addition, since the solar cell elements are serially connected, influences of partial shadows on the power generation efficiency also increases. For example, if one of the serially connected solar cell elements is covered with a partial shadow, the current generated from the cell is reduced and the rates of currents generated from other cells are also limited by this cell.
Reducing the influence of this partial shadow requires a bypass diode to be connected in parallel to each of the serially connected solar cell elements. However, even using this method cannot completely eliminate the influences of the partial shadow on other power generating cells.
Furthermore, as described in Japanese Patent Application Laid-Open No. H08-70533, providing an inverter for each solar cell element may alleviate work in the serial connection step which is the above described problem, but this requires a cutting step and an etching step at the ends of individual solar cell elements when creating those solar cell elements, which still takes time and trouble.
Furthermore, when a structure of mounting solar cell elements on a support is adopted, it is necessary to precisely locate those solar cell elements at certain intervals to improve electric insulation between the respective solar cell elements, the external appearance and efficiency of area power generation, which is however a difficult task and constitutes a factor responsible for a cost increase.
U.S. Pat. No. 4,773,944 discloses a solar cell module comprising individual solar cell elements, each of which is formed on a single substrate, all connected in parallel as a solution to all the above described problems such as the complicated step of serial connections, cost increase, influences of partial shadows and difficulty in mounting work.
This solar cell module is constructed in such a way that a current collection bus bar is connected to a current collection electrode of each solar cell element and a plurality of outputs of the solar cell elements are collected into a single output.
However, in such a configuration, the value of the current flowing through the current collection bus bar is the sum of the plurality of output currents of the solar cell elements, causing another problem that as the number of solar cell elements increases and the area of the solar cell module increases, loss in current collection also increases considerably.
To solve the problem of this current collection loss, the cross section of the current collection bus bar may be increased, but this solution causes the weight and volume of the current collection bus bar to grow considerably, making the producing/transporting work difficult.